(1) Field of the Invention
The present invention lies in the area of mechanical power transmission means and, more specifically, in the area of mechanical power transmission means intended for rotary-wing aircraft. Yet more specifically, the present invention relates to a damping device for a supercritical mechanical power transmission shaft. This damping device makes it possible primarily to dampen the vibrations of a supercritical shaft during transitions to critical rotation speeds.
(2) Description of Related Art
A rotary-wing aircraft usually includes at least one main rotor, ensuring the support and propulsion of the aircraft, and a rear rotor, ensuring primarily an anti-torque function in yaw. Such an aircraft includes a power plant that is equipped, for example, with at least one turbo motor and that mechanically drives a main power transmission gearbox. This main power transmission gearbox directly drives the main rotor rotationally. Meanwhile, the rear rotor is rotationally driven by a rear power transmission train that, in principle, is mechanically linked to the main power transmission gearbox.
This rear power transmission train traditionally includes at least two transmission shafts leading from the main power transmission gearbox, such as, for example, a front transmission shaft and a rear transmission shaft, as well as an auxiliary power transmission gearbox that mechanically links the rear transmission shaft and the rear rotor.
The coupling of these two transmission shafts may be subject to multiple defects, consisting primarily of angular misalignment, radial misalignment, and an axial offset between these shafts.
The use of a flexible coupling makes it possible to compensate for such an angular misalignment defect. A flexible coupling, usually designated by the term “flector”, consists, for example, of a stack of steel sheets. However, such an elastic coupling has no effect on radial misalignment or on an axial offset.
These transmission shafts may also be guided rotationally by bearings mounted on elastic rings that make it possible, on the one hand, to compensate for these defects, and, on the other hand, to dampen vibrations and/or the deformations of these transmission shafts. These bearings consist, for example, of ball bearings and elastic rings made of an elastomeric material.
In particular, the rear transmission shaft may be very long and may require the use of multiple intermediate bearings. Indeed, such a rear power transmission train requires the use of a large number of linking and guiding parts that may entail high cost and substantial mass. Furthermore, these defects, these vibrations, and these deformations may cause rapid deterioration of the bearings and/or of the elastic rings, thereby leading to frequent and expensive maintenance operations.
In order to eliminate the intermediate bearings and the linking parts, a long and flexible transmission shaft can be used. This shaft must be capable of withstanding significant rotational speeds imposed by the driving of the rear rotor of a rotary-wing aircraft. For example, such a transmission shaft may reach rotational speeds on the order of 2,000 revolutions per minute (2,000 rpm), or even 6,000 rpm in certain aircraft, with this transmission shaft being between 3 and 4 meters (3 and 4 m) long. In addition to the elimination of the linking and guiding parts, such a transmission shaft enables a substantial gain in terms of mass, including the mass of the shaft itself
Conversely, for such transmission shafts there are specific rotational speeds that generate significant deformations. Indeed, for such rotational speeds, the centrifugal forces resulting from an imbalance of the shaft cause gradually increasing flexing of the shaft, unless this phenomenon is attenuated. The elastic restoring forces that are generated when the shaft is deformed are smaller than the centrifugal forces that are generated by the flexed shaft. The flexing of the shaft then increases until it is limited by either the physical structure surrounding the transmission shaft or the characteristics of the shaft itself.
Such phenomena can occur as soon as the rotational speed of the shaft generates vibrations that are close to each frequency of the shaft. In fact, these phenomena can appear as soon as the rotational speed of the shaft is equal to a rotational speed that corresponds to each vibration mode of the shaft. Such rotational speeds of the shaft are defined, can be determined beforehand, and are known as “critical speeds”. Such a mechanical power transmission shaft is usually referred to as a “supercritical shaft”.
When such deformations occur, a supercritical shaft whose rotational speed is essentially equal to a critical speed has certain points along its length that do not undergo flexion (that is, they are not displaced transversely). These points are known as “nodes” or “nodal points”. Conversely, the points along the supercritical shaft that are displaced transversely with the greatest amplitude constitute the so-called “bellies” of this shaft.
It is known that, in order to limit the amplitudes of the supercritical shaft during the transition to critical speeds, damping devices can be installed at the location of these bellies or in proximity to them. When a supercritical shaft that has reached a critical speed starts to depart from its axis of rotation, such damping devices make it possible, on the one hand, to limit the flexion of the shaft and, on the other hand, to dissipate the energy of this deformation. The deformation of the supercritical shaft then diminishes as its rotational speed increases, actually moving away from the critical speed.
The supercritical shaft can then achieve a rotational speed that is essentially equal to a critical speed, although this transition should be merely temporary before a higher nominal rotational speed is reached. Care is simply taken to set the nominal rotational working speed of the supercritical shaft within a range of rotational speeds that is sufficiently far removed from its critical speeds corresponding to its individual vibration modes, so as to avoid the risk of generating vibrations that would be destructive for this supercritical shaft over the long term.
For example, if the nominal rotational speed of a supercritical shaft is located between the critical speeds that correspond to the first and second individual vibration modes of this shaft, then this shaft will encounter a single critical speed before reaching its nominal rotational speed. Similarly, if the nominal rotational speed of a supercritical shaft is located between the critical speeds that correspond to the third and fourth individual vibration modes of this shaft, then this shaft will encounter three critical speeds before reaching its nominal rotational speed.
In such a context, a known type of damping device includes a stationary support and a disc that are provided, respectively, with an annular opening through which the supercritical shaft passes. Springs press the disc against the support, with the disc being movable in a plane perpendicular to the axis of rotation of the shaft.
During the transitions to the critical speeds, the supercritical shaft, upon being deformed, comes into contact with the annular opening of the disc, with the disc then being displaced along with the shaft, which continues to be deformed. Because of friction between the disc and the support, as generated by the action of the springs, this displacement of the disc makes it possible to dissipate at least part of the energy of the deformation of the shaft. Furthermore, because the displacement of the disc is limited, the damping device also makes it possible to limit the amplitude of the deformation of the shaft.
The deformations of the supercritical shaft then diminish and disappear as the rotational speed of the shaft increases, thereby moving away from the critical speed. The disc is then re-centered in the damping device, drawn by the shaft that is re-centered around its axis of rotation.
However, a recurrent defect is encountered with this type of damping device. The supercritical shaft is in rotation when it comes into contact with the annular opening of the disc. In fact, as a result of this rotation, it induces a circular motion of the disc, which then generates an angular displacement of the disc with respect to the support around the axis of rotation of the critical shaft.
This angular displacement persists after the supercritical shaft has been re-centered around its axis of rotation, with no element of this damping device enabling the nullification of this angular displacement of the disc.
However, known document EP2418396 describes such a damping device that includes a mechanism for the complete re-centering of the disc. This damping mechanism is complex and includes an intermediate part as well as four springs. The four springs are positioned parallel to each other and perpendicular to the axis of rotation of the shaft, with two of the springs being located between the support and the disc, and two of the springs being located between the support and the intermediate plate.
Furthermore, document FR1054332 describes a bearing assembly for a rotating shaft that makes it possible, on the one hand, to oppose the oscillations and the rotational movements of the assembly around the rotating axis, and, on the other hand, to allow linear, circular, or elliptical oscillations of the bearings. In particular, these bearings include elastic means that are located in two different planes, with the said planes being perpendicular to each other.
Moreover, known documents FR2908735 and FR2908736 describe a magnetic damping device for a power transmission shaft in a helicopter. This device includes a magnetic bearing that damps the vibrations of the transmission shaft, with the said magnetic bearing being attached to the structure of the helicopter by means of a non-magnetic damper. This non-magnetic damper makes it possible to limit the radial oscillations of the magnetic damper and, consequently, those of the shaft. Document US2002/065139 also describes a magnetic damping device.
Last, document EP1918198 describes a damping device in which a damping element is located between two metal plates. Clamping assemblies consisting of a screw, a nut, and a spring make it possible to press these plates against the damping element.
Thus, the present invention relates to a damping device for a supercritical rotating shaft, which device is simultaneously simple, lightweight, and inexpensive, while enabling the effective damping of the deformations of the shaft and the correction of the angular displacement defect caused by the rotation of the shaft.